Release of waste industrial gases, landfill gases, and/or combustion products or byproducts to the atmosphere may be regulated for safety or to protect the environment and, therefore, have limits established on the quantity of one or more gases that may be released to the environment. Major components of landfill gas are methane, carbon dioxide and nitrogen. There are also minor amounts of oxygen, other organic compounds, halides and hydrogen sulfide. Landfill gases, which present a potential renewable energy source, are believed by many scientists to contribute to the greenhouse effect. At present, landfill gases are dealt with by venting to the atmosphere; burning in boilers; flaring; fueling gas turbine or internal combustion engine-driven generators; or as a fuel substitute with or without gas components other than methane.
Landfill gas is seen to be a potential source of carbon dioxide, and also as a potential source of natural gas. Several prior art techniques disclose processes for purification of landfill gas to recover carbon dioxide from landfill gas. Some of these processes also remove impurities such as sulfur compounds, halogens, water, and the like. Other processes involve treatment of landfill gas by converting hydrogen sulfide (H2S) to sulfur dioxide (SO2) in a burner. The sulfur dioxide is then transported to a bed of lime wherein the sulfur is deposited. A disadvantage of this system is that the sulfur containing byproducts can deposit in the transfer lines and can cause an increase in pressure within the system.
In other methods of controlling or reducing the quantity of some gases released, gases may be scrubbed of sulfur dioxide or nitrogen oxides (capture and removal of a gas is commonly referred to as scrubbing) to reduce the quantity released after the landfill gas is combusted. These methods may operate by causing a chemical reaction between the gas and other substances. For example, gases may be reacted with sulfur dioxide, calcium carbonate (or limestone), water, oxygen or mixtures thereof to form a dissolved or precipitated solid containing the captured sulfur, which can be collected using known techniques. In some instances gaseous pollutants, such as oxides of nitrogen, can be reacted with other gases or liquids to form new compounds including other gases or liquids. The reaction between nitrogen oxides, ammonia gas, or ammonium hydroxide solution can form nitrogen (N2) gas and water which can be released to the atmosphere without harm to the environment. In some cases, gases such as hydrogen sulfide (H2S) may be reacted with oxygen to form sulfur dioxide (SO2). In other cases, gases such as hydrogen sulfide (H2S) may be reacted with oxygen with the aid of sulfur oxidizing bacteria to form elemental sulfur, which can be captured and collected to reduce emissions.
All such methods rely on use of reactants such as limestone, ammonia, oxygen, or other chemicals to separate, capture, sequester, or convert the gas to be controlled into a form more suitable for recovery and sale, or for disposal. Most of these reactants form a substantial part of the cost for control of emissions, in addition to the high cost to cause the reactants and the gases to come into intimate contact with each other so that the desired reactions can take place.
Kinetic theories have predicted that a partial separation of constituents of a gaseous mixture will occur when the mixture is subjected to a pressure gradient. Industrial processes for separating individual fractions of mixtures on the basis of a pressure gradient are not widespread. In order to obtain sufficient separation between components of a gaseous mixture, relatively steep pressure gradients are required. In the past, large pressure gradients can be achieved in a gaseous mixture using a standard gas centrifuge. Other devices utilizing pressure diffusion sometimes include a separation nozzle, particularly for enrichment of isotopes of uranium.
Some methods of separation of gaseous mixtures into their component fraction use the differences in the molecular weights or density of the gases in the mixture to facilitate the separation. The difference in masses of the gases gives rise to a difference in diffusion rate, and this has been utilized in the separation or enrichment of isotopes, such as uranium. In these methods, a mixture of gases is impinged on a porous membrane in which the pores are small enough for lighter molecules to pass more readily through the pores than the heavier ones. The mixture can thus be separated by flowing it along the membrane surface; the portion that passes through is enriched as the lighter component, and the portion that does not is enriched as the heavier component.
In gas mixtures comprising uranium isotopes the density difference (using UF6 gas) between U235 and U238 is less than 1% of the total gases in the mixture. That is, UF6 gas with the heavier isotope is 1.00859 times heavier than the UF6 gas with the lighter isotope. Because the density difference in these uranium isotopes is small, several repetitions of the separation process are required to achieve the desired degree of separation. In contrast, the density differences between gases in pollutants and/or industrial waste gases are greater. For example: sulfur dioxide gas is approximately 3.5 times heavier than water vapor; sulfur dioxide gas is approximately 1.46 times heavier than carbon dioxide gas; carbon dioxide gas is approximately 2.74 times heavier than methane gas; and carbon dioxide gas is approximately 1.29 times heavier than hydrogen sulfide gas.
For gas mixtures with gases having larger differences in molecular weight or density, gas centrifuges have been used in separating these gases from gaseous mixture. Typical or standard gas centrifuge includes a tall vertical rotary cylinder fed with the gas mixture to be separated. The cylinder is rotated about its axis at a high angular velocity. The rotation of the cylinder causes the gas mixture to increase its angular rotational velocity so that the lighter components of the mixture move toward the axis and the heavier components of the mixture move toward the wall. Under standard conditions, significant high-purity separation is difficult to achieve unless the rotational velocity is extremely high. In another method for separation of gases, separation nozzles have been used. Separation nozzles induce a pressure gradient in a curved expanding supersonic jet to achieve separation of a gas mixture. The power consumption of separation nozzles is significant relative to the separation achieved. In centrifugal gas separation devices, such as countercurrent gas centrifuges and expanding jet or separation nozzle centrifuges, many stages cascaded together have often been required in order to obtain the desired separation.
Another device sometimes suggested for gas-gas separation includes a vortex tube or a vortex chamber separator in which a fluidic separation process results from centrifugal forces used for separating or precipitating a denser disperse phase from a lighter phase with continuous flow. Vortex chamber separators have the disadvantage of relatively lower separating efficiency relative to the energy requirement, primarily because of high flow resistance in the vortex chamber and also the use of multi-chamber systems with relatively high volume.
Consequently, a need exists for improvement in separation of gaseous components from gaseous mixtures in order to control emissions. There is a critical need for a cost-effective system for removing gaseous pollutant species from gaseous mixtures, such as landfill gas.